Boron depletion in Galactic early B-type stars reveals two different main sequence star populations

TL;DR

This study investigates boron depletion in Galactic early B-type stars to test stellar evolution models with rotational mixing, revealing two distinct star populations with different boron depletion mechanisms and evolutionary histories.

Contribution

The paper introduces a comprehensive grid of rotating stellar models with detailed nuclear networks and compares them to observations, identifying two star populations with different boron depletion origins.

Findings

Two-thirds of stars match models with rotational mixing efficiency of ~50%.

One-third of stars, especially slow rotators, are incompatible with current models.

A different boron depletion mechanism is needed for a subset of stars.

Abstract

The evolution and fate of massive stars are thought to be affected by rotationally induced internal mixing. The surface boron abundance is a sensitive tracer of this in early B-type main sequence stars. We test current stellar evolution models of massive main sequence stars which include rotational mixing through a systematic study of their predicted surface boron depletion. We construct a dense grid of rotating single star models using MESA, for which we employ a new nuclear network which follows all the stable isotopes up to silicon, including lithium, beryllium, boron, as well as the radioactive isotope aluminium-26. We also compile the measured physical parameters of the 90 Galactic early B-type stars with boron abundance information. We then compare each observed stars with our models through a Bayesian analysis, which yields the mixing efficiency parameter with which the star is reproduced the best, and the probability that it is represented by the stellar models. We find that about two-thirds of the sample stars are well represented by the stellar models, with the best agreement achieved for a rotational mixing efficiency of ~50% compared to the widely adopted value. The remaining one third of the stars, of which many are strongly boron depleted slow rotators, are largely incompatible with our models, for any rotational mixing efficiency. We investigate the observational incidence of binary companions and surface magnetic fields, and discuss their evolutionary implications. Our results confirm the concept of rotational mixing in radiative stellar envelopes. On the other hand, we find that a different boron depletion mechanism, and likely a different formation path, is required to explain about one-third of the sample stars. The large spread in the surface boron abundances of these stars may hold a clue to understanding their origin.